The present invention relates to fluid coupling devices, and more particularly, to fluid coupling devices of the type including modulating valving.
Although the present invention may be used advantageously in fluid coupling devices having many different configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Viscous shear couplings have been quite popular for many years for driving engine cooling fans, primarily because their use results in a substantial saving of engine horsepower, when compared to a conventional shaft driven fan. The horsepower savings results from the fact that the viscous coupling operates in the engaged, full speed condition only when cooling is needed, and operates in a disengaged, relatively lower speed condition when little or no cooling is required.
In an effort to effect even greater savings of engine horsepower, those working in the art developed "modulating valving" for fluid couplings of the type to which the invention relates. See U.S. Pat. No. 3,227,254, assigned to the assignee of the present invention. Briefly stated, modulating valving has the effect of moving the fluid inlet opening radially outward as the temperature increases, to progressively increase the volume of fluid in the operating chamber of the device as the ambient air temperature increases over a predetermined range.
As is well known to those skilled in the art, the relationship between the output speed of a viscous fluid coupling, and its input speed, is determined by the volume of fluid in the operating chamber which, in turn, is determined by the relationship of the fill and discharge rates. Once sufficient fluid has been forced into the operating chamber to yield the maximum output speed, in response to increasing temperature, it would be expected that a decreasing temperature would then cause fan speed to decrease, following the same speed vs. temperature curve as during the increasing temperature condition. However, it has been shown that this does not occur, but instead, a substantial drop in temperature (as much as 20 degrees F. or about 12 degrees C.) must occur before the fan speed begins to decrease. This phenomenon is referred to as "hysteresis", which will be described in greater detail subsequently. As is well known to those skilled in the art, some hysteresis is inevitable because of factors such as internal friction, etc., but excessive hysteresis is undesirable because it represents wasted engine horsepower, driving the fan at a speed higher than necessary to provide adequate cooling, at any given ambient air temperature.